This invention relates to a method of separating unburned Carbon, as particulate matter, from the flyash produced as a result of the combustion process occurring in a pulverized-coal fired steam generating power plant and more particularly to the separation of the flyash into a coarse product group and a fine product group utilizing the differences in specific gravity between the Carbon particles and remaining flyash as well as the use of a rotary classifier to effect the separation whereby at least one of the two product groups comprises a relatively low weight percentage of unburned Carbon while yet comprising a relatively high percentage of the total mass of product recovered after separation.
For use in a pulverized-coal fired steam generating power plant raw coal is usually crushed in a pulverizer to a very fine powder and then injected into the furnace of the steam generator where it is combusted in air. The combustion process may typically result in a rotating flame envelope of hot flue-gases called a "fireball." The hot flue-gases rise within the furnace giving up heat to the working fluid of a thermodynamic steam cycle to produce steam. The flue-gases then exit the furnace through a horizontal pass and are directed to and through a backpass. The horizontal pass and backpass commonly contain heat exchange surfaces which interact with the flue-gases to superheat and reheat the steam. The flue-gases then typically pass from the backpass to an air preheater where, in a heat exchange process, air is heated prior to use in the furnace combustion process. From the air preheater the flue-gases are directed to a flue-gas cleansing apparatus--usually an electrostatic precipitator, a fabric filter, a selective or nonselective catalytic reduction chamber or some combination thereof. Finally the flue-gases are directed to a stack and thence vented to the atmosphere.
Raw coal, as mined, typically contains, among other constituents, varying percentages of fixed Carbon, ash, volatile matter and water. However, as a result of the incomplete combustion of the coal constituents, the resulting flue-gases are laden with particulate matter; including unburned Carbon particles. Depending upon its nature, this combination of unburned Carbon and other particulate matter is generally referred to as bottom ash or flyash. Bottom ash is slag which builds up on the heat-absorbing surfaces of the furnace, superheater and reheater and falls, either by its own weight or as a result of load changes in the power plant or by sootblowing, to the bottom of the furnace where it is collected in a hopper. In contrast, flyash is carried along with the flue-gases in the flow path described above where it is collected at several points; namely at the exit to the backpass in an economizer ash hopper, after passing through the air preheater in an air heater ash hopper and in hoppers at the flue-gas cleansing apparatus.
The presence of the unburned Carbon particles in the flyash is due to, among other things, furnace design, the mineral content of the coal, the particle size of the pulverized-coal, the furnace temperature and the stoichiometry of the combustion process. If the flyash can be made to be less than 5% unburned Carbon by weight it becomes a commercially valuable byproduct. In particular it can be used beneficially as a cement and concrete admixture, as blasting grit, or in roofing materials, snow and ice control and structural landfills. However, if the unburned Carbon content of the flyash is above 5% the flyash has no commercial value and must be disposed, usually by expensive and environmentally hazardous landfilling methods.
Given that a typical utility boiler rated at 400 MWe firing a 10% ash coal may create over 90,000 tons of flyash per year it can be seen that if the flyash can be sold rather than disposed, the economic benefits to power plant owners can be substantial. Considerable efforts have been expended in the precombustion and combustion stages of pulverized-coal firing in attempts to maintain the resulting unburned Carbon in flyash at less than 5%. For instance, pulverizer design has been continuously improved to provide finer and finer particle sizes to in turn improve the combustion of coal, lower NO.sub.x emissions and reduce the content of unburned Carbon in flyash. The continued reduction in coal particle sizes can be seen through the evolution of whizzer classifiers, static classifiers and rotary classifiers. However, even the use of rotary classifiers may not reduce the levels of unburned Carbon in flyash to desirable levels. This could occur, for example, if the coal is of lower reactivity or if the operation of the pulverizer system limits the performance of the rotary classifier. In addition, it is illustrative that the subject matter of U.S. Pat. No. 5,315,939, entitled "Integrated Low NO.sub.x Tangential Firing System," issuing on May 31, 1994 and assigned to the same assignee as the present application, relates to a tangential firing system wherein the NO.sub.x emission target is to be achieved through combustion techniques only, while maintaining Carbon-in-flyash at less than 5% and CO emissions at less than 50 ppm.
Other methods to maintain the resulting unburned Carbon in flyash at desired levels have also been utilized. To that extent, representative of the prior is U.S. Pat. No. 5,024,169, entitled "Process To Refine Flyash Captured From Pulverized Coal Fired Boilers And Auxiliary Equipment" which issued on Jun. 18, 1991 and discloses a process and apparatus for the refinement of exhaust particulate matter from a boiler or other device burning an organic fuel such as coal. Further in the prior art is found U.S. Pat. No. 5,160,539, entitled "Method And Product Of Fly Ash Benefication By Carbon Burnout In A Dry Bubbling Fluid Bed" which issued on Nov. 3, 1992 and discloses a method of reducing Carbon content of fine particles of flyash containing Carbon. Also in the prior art, U.S. Pat. No. 5,399,194, entitled "Method Of Fly Ash Beneficiation And Apparatus For Same" and issuing on Mar. 21, 1995, discloses an invention which relates to beneficiation by oxidation of fine particles of carbonaceous flyash and more particularly beneficiation of fine particles of flyash having low incipient agglomeration temperatures.
It is also desirable to maintain the amount of unburned Carbon in the flyash by mechanical means. Such a method is simple, versatile and cost effective and may be incorporated in new power plant design or in retrofit applications.
As evidenced in the prior art, the need to separate particulate matter is not new. Typical methods for effecting the separation of particulate matter often utilize gravitational or centrifugal forces or some combination thereof. In particular, U.S. Pat. No. 580,145, entitled "Pneumatic Separator" and which issued on Apr. 6, 1897, teaches a separator in which the material to be separated is carried by a moving air-current and the separation is effected by gravity. Further in the prior art, U.S. Pat. No. 687,266, entitled "Pneumatic Separator" and which issued on Nov. 26, 1901, discloses separators employed for extracting the finer from the coarser particles of various pulverized materials, as ores, marble, gypsum, cements, sand, coal, etc., in which a diminishing force of an air-current acts against gravity. Still further, U.S. Pat. No. 688,810, entitled "Pneumatic Separator" and which issued on Dec. 10, 1901, teaches pneumatic separators of that class in which the material to be separated is carried by an air-current and the separation effected by gravity. Yet further in the prior art, U.S. Pat. No. 1,783,357, entitled "Mechanical Separator" and which issued on Dec. 2, 1930, teaches an improved means for insuring the separation from the rising air stream, of all of the heavier particles of material. Yet again in the prior art, U.S. Pat. No. 3,865,242, entitled "Upstream Classifier For A Multi-Separator" and which issued on Feb. 11, 1975, discloses a multi-separator processing organization for effectively classifying a flow stream containing particulate matter of various densities into plural fractions according to the density characteristics of each.
The use of centrifugal forces to effect the separation of particulate matter is also seen in the prior art. In particular U.S. Pat. No. 2,195,618, entitled "Adjustable Separator" and which issued on Apr. 2, 1940, relates to new and useful improvements in adjustable mechanical separators, and more particularly to improved means for grading or determining, within certain limits, the fineness of the material that is delivered from the separator. Also in the prior art, U.S. Pat. No. 3,770,124, entitled "Swing Back Whizzer Blades For Mechanical Air Separator" and which issued on Nov. 6, 1973, teaches a mechanical separator for selectively dividing an already finely divided solid into two products. Furthermore, U.S. Pat. No. 4,653,699, entitled "Coal-Mill Classifier" and which issued on Mar. 31, 1987, discloses a centrifugal air classifier, of the type particularly suitable for use with coal mills, comprising a coaxial course-material collecting hopper which is arranged in the truncated-cone-shaped classifier housing and into which the material to be classified enters with the carrier gas through a spin-generating ring of regulating flaps and from which the mixture of fine material and carrier gas emerges from the classifier through a coaxial baffle tube and the coarse material is removed through the open lower end of the collecting hopper.
The separation of particulate matter may also be accomplished, as alluded to above, by what are generally known as static classifiers and rotary classifiers. These types of classifiers are commonly seen in the art of pulverizing raw coal before its introduction into the furnace of a steam generating power plant for combustion therein. In a static classifier a flow of air, combined with coal particles entrained therein which have undergone a first pulverizing action, is directed through a series of stationary turning vanes which make up a convoluted flow path through a pulverizer. Said turning vanes are canted at an angle to the direction of the flow of the stream of air and coal particles so as to cause the coarsest (and therefore heaviest) particles to fall out of the air stream and suffer a second pulverizing action while the relatively finer coal particles pass through the classifier and are delivered to the furnace for combustion. U.S. Pat. No. 4,684,069 discloses a prior art static type of vertical mill.
In a rotary classifier the flow of air, combined with the initially pulverized-coal particles entrained therein, is directed through a series of squirrel cage like vanes disposed as an inverted, truncated cone revolving about the central vertical axis of a pulverizer housing at a predetermined rotational velocity. The vanes are canted at an angle to the direction of the flow of the stream of air so as to present to the stream a "window" through which the stream of air and certain coal particles may pass unimpeded. The width of the window is variable in that as the rotational velocity of the vanes is increased the window is made narrower and as the rotational velocity of the vanes is decreased, the window is made wider. The rotational velocity of the vanes coupled with the velocity of the air stream acts to separate the coal particles into two groups. A first group of particles are those that are relatively coarse or heavy and therefore moving too slowly to pass unimpeded through the aforesaid window. These particles collide with the vanes and fall out of the air/coal stream and are returned to suffer a second pulverizing action. A second group of particles are those that are relatively fine or light and are therefore moving fast enough to pass unimpeded through the window and thus through the pulverizer to the furnace of the steam generator for combustion therein. For a fixed velocity of the air stream, by the judicious manipulation and control of the aforesaid rotational velocity of the vanes, the relative fineness of the two groups of coal particles may be adjusted, i.e., by increasing the rotational velocity of the vanes, the fineness of the coal particles that pass through the aforesaid window increases. In other words, only finer and finer particles are capable of passing unimpeded as rotational velocity increases and while yet coarser and coarser coal particles are capable of passing unimpeded as rotational velocity is reduced. Conversely, for a fixed rotational velocity of the vanes, by the judicious manipulation and control of the aforesaid velocity of the air stream, the relative fineness of the two groups of coal particles may again be adjusted, i.e., by increasing the velocity of the air stream, the fineness of the coal particles that pass through the aforesaid window decreases. In other words finer and finer particles will pass unimpeded as air velocity decreases and coarser and coarser coal particles will pass unimpeded as air velocity is increased. In this regard U.S. Pat. No. 2,092,310, entitled "Rotary Classifier For Pulverizers And The like" and which issued on Sep. 7, 1937, relates to rotary classifiers adapted for use in material pulverizers in which the pulverized material is removed by a gaseous carrier medium. Furthermore, U.S. Pat. No. 4,684,069 also discloses a prior art rotary blade type of vertical mill. Still further in the prior art, U.S. Pat. No. 5,622,321, entitled "Mill Classifier" and which issued on Apr. 22, 1997, relates to a mill classifier, sifter or separator and in particular a roller mill classifier having a static classifier and a dynamic classifier and an annular classifying zone formed between these two classifiers, in which the static classifier is constituted by a radially outward positioned distributor having guide blades and the dynamic classifier is constituted by a ledge rotor. Also in the prior art U.S. Pat. No. 5,624,039, entitled "Separator For Sorting Particular Material" and which issued on Apr. 29, 1997, relates to a separator for sorting of particulate material suspended in a conveying gas into a fine fraction and a coarse fraction.
Thus, although methods to effect the separation of particulate matter, such as those practiced in accordance with the teachings of the above referenced U.S. patents, have been demonstrated to be operative for the purpose for which they have been designed, i.e., the separation of particulate matter by size, there has nevertheless been evidenced in the prior art a need for such particle separation methods to be improved.
In particular it is desirable to be able to effect the separation of particulate matter based upon particle type as opposed to simply particle size. The current invention proposes to mechanically separate particulate matter based upon particle type, and in particular to separate unburned Carbon particles from a collection of unburned Carbon particles and other particulate matter, collectively known as flyash, at prescribed locations along the flue-gas flow path described above. The separation is such that a fine product group and a coarse product group are produced wherein the coarse product group contains a relatively small percentage by weight of unburned Carbon while yet comprising a relatively high percentage of the total mass of product recovered after separation. The separation is accomplished by taking advantage of the differences in the specific gravities of the unburned Carbon and remaining flyash in conjunction with the use of the dynamics of a rotary classifier. The unburned Carbon particles are significantly lower in specific gravity than the remaining flyash and are commonly of a different size range. Flyash material, which includes unburned Carbon particles, is entrained within and conveyed by way of a fluid stream, at a controllable mass flow rate, into a separator housing and accelerated upwards through an annular opening of relatively small cross sectional area created by a collection cone disposed within the separator housing. Next, upon entering an expansion chamber of greater cross sectional area, the larger and heavier particles entrained within the fluid stream fall by gravity into the collection cone due to an abrupt decrease in the velocity of the fluid stream. These particles exit the separator as a coarse product group. The lighter, smaller particles continue to be conveyed with the fluid stream upwards toward a rotary classifier revolving at a controllable rotational velocity. Only the lighter Carbon particles and very small flyash particles, having sufficient velocity, pass through the aforesaid window of the rotary classifier to become a fine product group. The balance of these particles, i.e., the larger Carbon and middling size flyash particles, lacking sufficient velocity, are struck by the rotating classifier vanes and thereby rejected. These particles in turn fall into the collection cone and become part of the coarse product group.
It is therefore an object of the present invention to provide a new and improved method of separating particulate matter based upon differences in the specific gravity of the particulate matter.
It is also an object of the present invention to provide a new and improved method of separating particulate matter based upon differences in the velocity of the particulate matter in a fluid stream.
In particular, it is an object of the present invention to provide a new and improved method of separating unburned Carbon particles from the collected flyash produced as a result of the combustion process occurring in a pulverized-coal fired steam generating power plant.
It is also an object of the present invention to provide a new and improved method of separating unburned Carbon particles from the collected flyash produced as a result of the combustion process occurring in a pulverized-coal fired steam generating power plant when not able to do so by boiler efficiency alone.
It is yet a further object of the present invention to provide such a new and improved method of separating the unburned Carbon particles from the collected flyash produced as a result of the combustion process occurring in a pulverized-coal fired steam generating power plant such that through the use thereof there are derived a first product group and a second product group.
It is still further an object of the present invention to provide such a new and improved method of separating the unburned Carbon particles from the flyash produced as a result of the combustion process occurring in a pulverized-coal fired steam generating power plant such that through the use thereof at least one of the product groups contains a relatively small weight percentage of unburned Carbon while yet comprising a relatively high percentage of the total mass of product recovered after separation.